Heat is transferred through a wall structure by the mechanisms of conduction, convection, and radiation. These mechanisms are introduced and developed in terms of their thermal resistances. Temperature difference is identified as the cause of heat flow through the structure which is impeded by the thermal resistances of the structures. Calculations are made of the thermal resistances at several points in a specific test wall section. The performance predicted from these calculations is compared to thermo-graphic measurements made on the wall under laboratory controlled conditions. These comparisons are used to draw conclusions as to the usefulness and limitations of thermographic practices.